Every material has molecular and electronic energy states which characterize its composition. These energy states are typically defined in terms of the interior or bulk of the material. In the real world, all materials have surfaces with dangling atomic bonds which produce molecular and electronic energy states, called surface states, different than those present in the underlying bulk material.
The energy states at the surface of a sample are dependent upon the structure and the stoichiometry of the underlying material as well as on the actual physical and chemical conditions on the surface of the material. The portion of the surface states which is a function of the underlying material only is called the intrinsic surface states. The portion of the surface states which arises from other physical and chemical conditions on the sample surface is called the defect surface states. Typically, the defect surface states arise from surface contamination or from strucutral defects in the surface structure of the sample. The surface state of almost any sample will be affected to some degree by defect surface states in addition to the intrinsic surface states.
The importance of the existence of surface states is extremely significant in the semiconductor industry. As can be appreciated, it is desirable to manufacture samples that are as free from contaminants and surface defects as possible. Unfortunately, it is extremely easy to introduce such surface defects into a sample. For example, it has been shown that when silicon wafers are polished or etched, additional surface defect states are introduced. These defect states can adversely alter the performance of a semiconductor device. The subject invention is designed to provide a new and convenient means for detecting that portion of the surface states which are attributable to defect surface states. In this manner, contaminants and surface characteristics of a sample can be evaluated.
Considerable effort has been made in the past to develop methods for detecting and evaluating surface states. A very complete discussion of semiconductor surfaces as well as a number of methods for detecting these states can be found in the reference Semiconductor Surfaces by A. Many, North Holland Publishing Company, Amsterdam, 1971. As set forth in this reference, surface states can be evaluated by measuring the electric field effects at the surface. Techniques for measuring sample conductance and capacitance have been utilized. Surface states have also been measured through magnetic detection techniques. Most of these techniques are inadequate since they tend to be contact technologies and only yield information on intrinsic surface states.
Optical techniques have also been utilized. One technique, called a tunneling microscope, is described in "Real-Space Observation of pi-Bonded Chains and Surface Disorder on Si(111)2.times.1", Feenstra, et al., American Physical Society, Vol. 56, No. 6, Feb. 1986. Another technique, known as spectroscopic ellipsometry, requires the use of a polarized beam which is reflected off the surface of the sample. By measuring the change in angle of polarization of the reflected beam, some information can be derived regarding the surface states. A description of ellipsometry techniques can be found in the following articles by Aspnes, "Microstructural Information from Optical Properties in Semiconductor Technologies", and "Optical Detection and Minimization of Surface Overlayers on Semiconductors Using Spectroscopic Ellipsometry." Both of these articles are published in SPIE, Vol. 276, Optical Characterization Techniques for Semiconductor Technology, at pages 188 and 227 (1981). (See also "Anisotropies in the Above-Band-Gap Optical Spectra of Cubic Semiconductors," Aspnes, The American Physical Society, Vol. 54, No. 17, p. 1956, April 1985.)
Another tool for evaluating surface states is through a photoacoustic technique. The use of photoacoustic spectroscopy to study optical absorption of surface states is described in detail in Photoacoustics and Photoacoustic Spectroscopy, by Allan Rosencwaig, John Wiley & Sons, 1980. In a photoacoustic technique, an intensity modulated heating beam is directed to the surface of a sample. The absorption of the heating beam is then measured as the wavelength of the heating beam is varied. In the earliest experiments, the sample was sealed in a gas-filled cell. Absorption is monitored by detecting sonic vibrations in the gas with a microphone.
The latter technique is suitable for spectroscopic analysis of surface states on a relatively large spatial scale. More specifically, in order to get the wide wavelength range of the heating beam needed for spectroscopic analysis, it is necessary to use an incandescent light source which is modified by a monochrometer. The incandescent source cannot be focused to less than one millimeter in resolution.
In the technique described above, spectroscopic information is obtained about the combined intrinsic and defect surface states. In order to separate the intrinsic surface states from the defect surface states, further analysis is required. In this analysis, the change in phase of the output signal is monitored as the modulation or chopping frequency of the heating beam is varied. Unfortunately, this approach is very difficult to achieve in practice since the phase changes are extremely small.
Another spectroscopic technique is described in "Temperature Dependence of the Si and Ge(111) 2.times.1 Surface-State Optical Absorption," Olmstead and Amer, American Physical Society, Vol. 33, No. 4, page 2564, February 1986. In this case, periodic deviations of a probe beam reflected off the surface of the sample are studied to detect optical absorptions.
Electroreflectance and photoreflectance spectroscopy has also been used to evaluate surface states. In these techniques, the surface of a semiconductor is periodically excited either with an alternating electric field (electroreflectance) or an intensity modulated beam of light (photoreflectance). Changes in reflectivity in the sample due to this periodic excitation are then monitored. (See, for example, "Effect of Surface and Nonuniform Fields in Electroreflectance: Application to Ge," Del Sole and Aspnes, Physical Review, Vol. 17, No. 8, page 3310, April 1978, and "Surface Studies by Modulation Spectroscopy," Heiland and Monch, Surface Science, Vol. 37, page 30, 1973).
None of the above techniques combines all of the desired qualities of a practical analytical tool for the manufacturing environment. Therefore, it would be desirable to provide an alternate technique for detecting defect surface states which is quick, easy to operate, highly sensitive and has high spatial resolution capabilities.
Accordingly, it is the object of the subject invention to provide a new and improved method and apparatus for evaluating surface states.
It is another object of the subject invention to provide a new and improved method for isolating defect surface states from intrinsic surface states.
It is a further object of the subject invention to provide an apparatus for detecting defect surface states with micron scale spatial resolution.
It is still another object of the subject invention to provide an apparatus which will provide a semiconductor manufacturer with the ability to characterize the surface contamination and surface damage of silicon or gallium arsenide which might arise from various integrated circuit manufacturing processes.